pubs.acs.org/Langmuir © 2010 American Chemical Society
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Particulate Mobility in Vertical Deposition of Attractive Monolayer Colloidal Crystals Kwan Wee Tan,*,†,‡,^ Yaw Koon Koh,§ Yet-Ming Chiang,†,‡ and Chee Cheong Wong*,†, †
Advanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, Singapore 117576, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, §DSO National Laboratories, Singapore 118230, and School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. ^ Present address: Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. )
‡
Received November 23, 2009. Revised Manuscript Received January 12, 2010 In the colloidal self-assembly of charged particles on surfaces with opposite polarity, disorder often dominates. In this report, we show that ionic strength, volume fraction, and solvent evaporation temperature can be optimized in the vertical deposition method to yield hexagonal close-packed monolayer arrays with positively charged colloids on negatively charged bare glass. We further extend our study to form well-defined binary two-dimensional superlattices with oppositely charged monolayers grown layer-by-layer. Our results suggest that the lack of particulate mobility in oppositely charged systems is the main cause of disorder, and maximum mobility is attained when all three growth parameters are finely adjusted to increase the time scale for the particles to stabilize and order during crystal growth in these attractive systems. A clear understanding and control of the collective behavior of highly mobile colloids could lead to the creation of greater diversity of nanoarchitectures.
1. Introduction Colloidal crystals have been studied extensively over the past decade for modeling of atomic systems1,2 and many potential applications,3 for example, lasing in ZnO inverse opal,4 water vapor sensing,5 and solar cell performance enhancement6 based on photonic band gap properties and as hypersonic phononic materials.7 In particular, two-dimensional (2D) ordered colloidal superlattices have received considerable attention for use in microlens arrays8 as well as templates9 to grow nanopillars10,11 and superhydrophobic nanoarrays.12,13 However, conventional colloidal self-assembly techniques based on repulsive interactions to form close-packed crystalline arrays severely limit the crystal structures achievable. Ionic colloidal crystals provide an *Corresponding authors. E-mail:
[email protected] (K.W.T.), wongcc@ ntu.edu.sg (C.C.W.).
(1) van Blaaderen, A. Science 2003, 301, 470–471. (2) Poon, W. Science 2004, 304, 830–831. (3) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693–713. (4) Teh, L. K.; Wong, C. C.; Yang, H. Y.; Lau, S. P.; Yu, S. F. Appl. Phys. Lett. 2007, 91, 161116–3. (5) Colodrero, S.; Oca~na, M.; Gonzalez-Elipe, A. R.; Mı´ guez, H. Langmuir 2008, 24, 9135–9139. (6) Yip, C.-H.; Chiang, Y.-M.; Wong, C. C. J. Phys. Chem. C 2008, 112, 8735– 8740. (7) Cheng, W.; Wang, J.; Jonas, U.; Fytas, G.; Stefanou, N. Nat. Mater. 2006, 5, 830–836. (8) Kumnorkaew, P.; Ee, Y.; Tansu, N.; Gilchrist, J. F. Langmuir 2008, 24, 12150–12157. (9) Li, Y.; Cai, W.; Duan, G. Chem. Mater. 2008, 20, 615–624. (10) Burmeister, F.; Badowsky, W.; Braun, T.; Wieprich, S.; Boneberg, J.; Leiderer, P. Appl. Surf. Sci. 1999, 144, 461–466. (11) Cheung, C. L.; Nikolic, R. J.; Reinhardt, C. E.; Wang, T. F. Nanotechno logy 2006, 17, 1339–1343. (12) Pacifico, J.; Endo, K.; Morgan, S.; Mulvaney, P. Langmuir 2006, 22, 11072– 11076. (13) Kawai, T.; Suzuki, M.; Kondo, T. Langmuir 2006, 22, 9957–9961. (14) Maskaly, G. R.; Garcı´ a, R. E.; Carter, W. C.; Chiang, Y.-M. Phys. Rev. E 2006, 73, 011402. (15) Bartlett, P.; Campbell, A. I. Phys. Rev. Lett. 2005, 95, 128302. (16) Leunissen, M. E.; Christova, C. G.; Hynninen, A.; Royall, C. P.; Campbell, A. I.; Imhof, A.; Dijkstra, M.; van Roij, R.; van Blaaderen, A. Nature 2005, 437, 235–240.
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alternative approach to introduce a diversity of complex structures by fine-tuning the electrostatic interactions between the nanoparticles.14-18 More recently, layer-by-layer growth of binary 2D superlattices with LS, LS2, and LS6 stoichiometries consisting of two oppositely charged large (L) and small (S) colloids stabilized by attractive electrostatic interactions have also been demonstrated.19,20 The key to engineering periodic order in colloidal systems is by understanding the collective behavior of the particles in an environment of high mobility. The incorporation of strongly electrostatic attractions further complicates the principles of the self-assembly mechanism. Via real-time microscopic video recordings, Yan et al.21 observed the formation of highly disordered packing of negatively charged polystyrene colloids on top of positively charged substrates, and vice versa. They attributed this phenomenon to the strong attraction between the dissimilarly charged substrate and particles which restrict the ordering process. On the other hand, colloids sharing the same polarity with the substrate are highly mobile to form well-ordered arrays. Ray and co-workers22 reported the formation of regularly spaced linear bands of 200-400 nm positively charged particles on negative glass and silicon substrates parallel to the air-substrate-solvent contact line. The deposition of these particulate bands is attributed to strong electrostatic adhesion to the substrate in a periodic stick-slip motion. A nearly hexagonal closed-packed monolayer was reportedly observed using 0.02 wt % latex (17) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59. (18) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420–424. (19) Tan, K. W.; Li, G.; Koh, Y. K.; Yan, Q. F.; Wong, C. C. Langmuir 2008, 24, 9273–9278. (20) Sharma, V.; Yan, Q. F.; Wong, C. C.; Carter, W. C.; Chiang, Y.-M. J. Colloid Interface Sci. 2009, 333, 230–236. (21) Yan, Q. F.; Gao, L.; Sharma, V.; Chiang, Y.-M.; Wong, C. C. Langmuir 2008, 24, 11518–11522. (22) Ray, M. A.; Kim, H.; Jia, L. Langmuir 2005, 21, 4786–4789.
Published on Web 01/25/2010
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Tan et al. Table 1. Experimental Parameters for Attractive Monolayer Colloidal Crystal Deposition
S/N
d of PS (nm)
vol (φ, %)
KCl conc (ci, μM)
temp (C)
morphology remarks
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3
371
0.5
0 10 1000 0 10 1000 0 10 1000 0 10 1000 0 10 1000 0 10 1000
35
linear bands of multilayers; hcp and random arrays linear bands of multilayers; hcp and random arrays linear bands of multilayers; hcp and random arrays monolayer and submonolayer arrays; locally ordered domains monolayer and submonolayer arrays; locally ordered domains multilayers and submonolayer arrays; aggregates and locally ordered domains monolayer and submonolayer arrays; locally ordered domains monolayer and submonolayer arrays; locally ordered domains submonolayer arrays, random aggregation monolayer and submonolayer arrays; short-ranged hcp domains monolayer and submonolayer arrays; short-ranged hcp domains submonolayer arrays, random aggregation monolayer and submonolayer arrays; long-ranged hcp arrays monolayer and submonolayer arrays; short-ranged hcp domains monolayer and submonolayer arrays; less random aggregation monolayer and submonolayer arrays; short-ranged hcp domains monolayer and submonolayer arrays; long-ranged hcp arrays monolayer and submonolayer arrays; less random aggregation
0.01 0.1 0.05 0.03 0.01
25
colloidal suspensions.22 Gray and Bonnecaze23 varied the bulk concentration, ionic strength, and zeta potential of the particles and substrate in a colloidal adsorption simulation study and concluded that the kinetics and structure formation on the surface are highly dependent on the energetic interactions between the oppositely charged particles and substrate. In this study, we explore the combined effects of electrostatic interactions, volume fraction, and evaporation temperature as tools to control the mobility of positive colloids to form well-defined hexagonal close-packed (hcp) monolayer arrays on negative bare glass substrates. We further demonstrate the optimization of the particulate mobility to enhance the quality of the attractive binary 2D superlattices on an oppositely charged colloidal template. A model of colloidal self-assembly in the presence of electrostatic attractive forces is also proposed.
2. Experimental Section 2.1. Polystyrene Colloids. Polystyrene (PS) colloidal spheres with diameters 604 and 371 nm (referred to as L and S particles) were synthesized using the emulsifier-free emulsion polymerization method.24 In order to obtain particles with both positive and negative surface charges, amindine- and sulfate-type initiators were used, respectively. The negatively charged 604 nm L-PS colloids have a zeta potential ζL = -36.1 mV, and the 371 nm S-PS are positively charged with zeta potential ζS = þ51.5 mV. The PS spheres were then dispersed in deionized water to form suspensions of specific volume fractions detailed in Table 1. 2.2. Fabrication of PS Monolayer Film. The positive PS colloidal monolayer was grown on top of borosilicate glass microslides (Menzel-Glazer) at 25 and 35 C using the flowcontrolled vertical deposition (FCVD) method as described elsewhere.25 Briefly, the colloidal suspension was contained in a glass vessel connected to a water bath to maintain a constant evaporation temperature condition. The glass microslides were held vertically to the inner walls of the container. The suspension was then drawn out from the vessel by a variable-flow peristaltic minipump (Fisher Scientific) which controlled the solvent meniscus falling velocity. In our experiments, the peristaltic minipump was drawing out the colloidal suspension at the maximum speed which was determined to be 0.5794 μm/s. The glass microslides were cleaned using piranha solution and then rinsed profusely with deionized water before use. (Warning! Piranha (23) Gray, J. J.; Bonnecaze, R. T. J. Chem. Phys. 2001, 114, 1366–1381. (24) Shim, S. E.; Cha, Y. J.; Byun, J. M.; Choe, S. J. Appl. Polym. Sci. 1999, 71, 2259–2269. (25) Zhou, Z.; Zhao, X. S. Langmuir 2004, 20, 1524–1526.
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solution reacts violently with organic materials. Handle with caution.) The experiments were conducted under ambient conditions of 23 C and relative humidity of 67%. Potassium chloride (KCl, Fisher Scientific) was added as the electrolyte to adjust the ionic strength in the suspension. 2.3. Fabrication of Binary PS Colloidal Film. The procedure to fabricate the binary colloidal specimen using FCVD has been described elsewhere.19 Briefly, the first monolayer of negatively charged L colloids (φL = 0.5 vol %) was grown on borosilicate glass microslides (Menzel-Glazer) at 35 C. The second layer of positive S-PS colloids (φS = 0.5 vol %) was grown on top of the previously formed L-monolayer to form a bilayer of oppositely charged colloidal film at three different temperatures: 25, 30, and 35 C. Potassium chloride (10 μM KCl, Fisher Scientific) was added as the electrolyte and nonionic surfactant, polyoxyethylene nonyl phenol (40 μM Igepal CO-720, SigmaAldrich), was added to reduce the surface tension of the PS suspension. 2.4. Characterization. The zeta potential of the particles was measured with Malvern Instruments Zetasizer Nano ZS90. The morphologies of the colloidal films were imaged with an Olympus BX51 optical microscope and a JEOL JSM 6340 field-emission scanning electron microscope.
3. Results and Discussion 3.1. Effect of Volume Fraction and Ionic Strength. When the positively charged PS colloids are deposited on the negative glass surface, linear patterns with widths up to 0.1 mm are formed for suspensions with volume fraction φ = 0.5 vol % at all ionic conditions (series A), similar to those observed by Ray and co-workers.22 The images in Figure 1 and Figure S1 in the Supporting Information show these 1D linear particulate bands are spaced apart at relatively regular intervals and deposited uniformly across the entire borosilicate glass substrates. This is an advantage of the FCVD which maintains a constant concentration of the suspension throughout the deposition process.25 The phenomenon has been explained as a result of the interplay of strong colloid-substrate electrostatic attractions and the stickslip motion of the meniscus growth front.22,26-28 First, because of the opposite surface charges, the positive colloids are attracted and adhered to the negative glass substrate. (26) Teh, L. K.; Tan, N.; Wong, C. C.; Li, S. Appl. Phys. A: Mater. Sci. Process. 2004, 81, 1399–1404. (27) Huang, J.; Tao, A. R.; Connor, S.; He, R.; Yang, P. Nano Lett. 2006, 6, 524– 529. (28) Thomson, N. R.; Bower, C. L.; McComb, D. W. J. Mater. Chem. 2008, 18, 2500.
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Figure 1. (a) Optical micrograph of linear positive PS patterns present in all three ionic conditions for φ = 0.5 vol % at 35 C (series A). The inset panels (b) and (c) SEM micrographs show the close-packed ordering of the colloids within each band.
The surface charge density of silica glass was determined to be -0.32 mC/m2.29 With such electrostatic attractions, the particles are likely to be laterally immobilized before they reach the contact line of the meniscus. This could also explain the disordered random ordering of charged particles “stuck” in the open spaces between the colloidal bands. As the meniscus recedes and deforms, a flux of spheres is delivered to the “pinned” contact line where the large attractive potential well of the glass substrate is screened, and interparticle distance becomes short enough for mutual interaction and ordering to take place in the suspension phase. This meniscus growth forms a thin band of multilayered colloidal crystals with hcp orientation. When the solution recedes further, the contact line “slips” rapidly to a lower “pinning” level, and the process is repeated. No hcp-oriented monolayers were observed in the series A specimens. This could be due to the particle concentration being too high, thus restricting lateral mobility necessary for the particles to order. However, when the colloidal concentration was reduced to 0.1 vol % (series C), the linear colloidal multilayered bands were replaced by monolayers with locally ordered configurations. Volume fraction has been credited to play a crucial role in crystalline formation by controlling the particle flux and thus the crystalline quality.23,25,30-34 A lower volume fraction leads to a smaller number of particles being transferred to the leading edge of crystal growth.30 Therefore, keeping evaporation temperature, ionic strength, and all other parameters constant, more time is required for the colloids to reach the threshold volume fraction for crystallization, implying that particles have a longer time to self-assemble in an orderly manner. Moreover, the average interparticle distance is increased and frequency of interparticle (29) Behrens, S. H.; Grier, D. G. J. Chem. Phys. 2001, 115, 6716–6721. (30) Kuai, S. L.; Hu, X. F.; Hache, A.; Truong, V.-V. J. Cryst. Growth 2004, 267, 317–324. (31) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132–2140. (32) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303–1311. (33) Kim, M. H.; Im, S. H.; Park, O. O. Adv. Funct. Mater. 2005, 15, 1329–1335. (34) Yan, Q. F.; Zhou, Z.; Zhao, X. S. Langmuir 2005, 21, 3158–3164.
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collisions is reduced. Thus, irreversible aggregation and random adsorption of positively charged colloids on the charged surface occurs more slowly compared to the ordering process, effectively imparting greater in-plane colloidal mobility to order. Electrostatic interactions play a key role in granting mobility to the particles to self-organize into regular crystalline arrays.14,19,23,35 With increasing ionic strength, the Debye screening length of the electrical double layers surrounding each positive sphere decreases. The consequence is twofold. First, the positive PS particles are able to approach one another closer and configure into a stable in-plane ordered array. And second, the shorter Debye length of the electrostatic attraction also implies the positively charged colloids have additional time to form the ordered array before settling onto the glass surface. Concurrently, as the volume fraction was reduced for each series of specimens, we added KCl (1:1 electrolyte) to change the ionic strength and electrostatic potentials to influence particle mobility and promote ordering in these attractive systems. The three ionic conditions are referred to as deionized (0 μM KCl), low ionic strength (10 μM KCl), and high ionic strength (1000 μM KCl). We observe both monolayer and submonolayer arrays with small locally ordered domains at low ionic strengths when φ = 0.1 vol % (series C). When the KCl concentration is increased to 1000 μM, the charged colloids become unstable and agglomerate rapidly, losing all mobility to self-assemble as shown in the Supporting Information. Interestingly, for φ = 0.05 vol % (series D), the local ordering of the positively charged colloidal spheres took the form of hcp-oriented arrays in small domains at low ionic strengths. We also observe long-ranged monolayer areas of compact hcp arrays in series E and F at lower volume fractions and low KCl concentrations. Figure 2 shows the SEM micrographs and corresponding fast Fourier transform (FFT) insets of hcp-oriented symmetry. (35) Rugge, A.; Tolbert, S. H. Langmuir 2002, 18, 7057–7065.
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Figure 2. Hexagonal close-packed domains and corresponding FFT inset panels at 25 C of (a) D2 [φ = 0.05 vol % and 10 μM KCl], (b) E1 [φ = 0.03 vol % and 0 μM KCl, and (c) F2 [φ = 0.01 vol % and 10 μM KCl].
The highest density of long-ranged hcp domains are observed in F2 of φ = 0.01 vol % with 10 μM KCl at 25 C. The FFT inset panel in Figure 2c displays the distinctive 6-fold-coordinated spots of a hexagonal lattice prominently and the most number 7096 DOI: 10.1021/la904435j
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Figure 3. SEM micrographs for φ = 0.01 vol % at 35 C of (a) B1, (b) B2, and (c) B3 in the ascending order of ionic strength. The ordering process was disturbed by the faster crystal growth rate and greater kinetic energies of the positive PS colloids at a higher evaporation temperature.
of diffraction orders (sharp rings). We postulate that with the progressive decrease of volume fractions and low ionic strengths the charged colloids attain an intricate balance between the Langmuir 2010, 26(10), 7093–7100
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Figure 4. Phase behavior of the positively charged polystyrene colloids on a negative borosilicate glass substrate at 25 C. The dashed lines indicate the approximate boundaries between the ordered and glassy phases at different conditions. The ionic strength axis is in logarithm scale.
particle-particle repulsion and particle-substrate attraction where they are sufficiently far apart to reorient into the geometrically and thermodynamically favored in-plane hexagonal close-packed lattice arrangement.36 3.2. Effect of Evaporation Temperature. The evaporation temperature is also a parameter that affects the particulate mobility by controlling both the particle flux moving to the growing colloidal clusters as well as the kinetic energy of the colloids to explore possible thermodynamically favorable lattice sites.30,37-40 However, over a certain high evaporation temperature, the rate of evaporation and hence the rate of crystal growth may be too fast for the particles to shift to favorable lattice sites and result in more defects.30,38,39 As mentioned earlier, we have successfully assembled hcpordered monolayer colloidal arrays under sufficiently low volume fractions (φ e 0.05) and ionic strength conditions (ci e 10 μM) at 25 C. Figure 3 shows that when the evaporation temperature was raised to 35 C in series B (φ = 0.01 vol %), aggregates and submonolayers with some local ordering morphologies are observed. We speculate the higher temperature increases both particle flux speed and a higher crystal growth rate such that the particles are unable to get into order before drying up. In addition, the associated higher kinetic energies of the spheres increase Brownian free motion and may have complicated the reorganization into the favorable close-packed arrays. It is worthwhile to mention that while lower temperature is preferred in our experimental conditions, the opposite holds true for particles and substrate of the same polarity, where better colloidal crystalline quality has been reported for elevated evaporation temperatures.30,37-39 Our observations of the various morphologies of positive PS colloids on negative glass are summarized in Table 1 and (36) (37) 52. (38) (39) (40)
Woodcock, L. V. Nature 1997, 385, 141–143. Ye, Y.; LeBlanc, F.; Hache, A.; Truong, V.-V. Appl. Phys. Lett. 2001, 78, Im, S. H.; Park, O. O. Langmuir 2002, 18, 9642–9646. R€odner, S. C.; Wedin, P.; Bergstr€om, L. Langmuir 2002, 18, 9327–9333. Cong, H.; Cao, W. Langmuir 2003, 19, 8177–8181.
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presented in a phase diagram when the temperature is at 25 C. We see in Figure 4 that hcp lattices are preferred for low volume fraction (φ e 0.05 vol %) and ionic strengths (ci e 10 μM KCl). However, under the same ionic strength conditions, a higher volume fraction could have introduced too many particles to the meniscus edge and impeded ordering by increasing collisions and reducing mobility. And at higher ionic strengths, the electrostatic repulsion range of the positive spheres is further reduced and agglomeration occurs. The role of particulate mobility in the crystal formation mechanism would be explained further in the next section. We have demonstrated that a fine control of the PS colloidal mobility would allow us to achieve single-component ordered structures. However, these close-packed lattices have limited crystalline symmetries, and their associated properties are too restricted for the diverse potential applications of colloidal crystals.41 Hereinafter, we extend our understanding of particulate mobility to explore more complex crystal architectures. In our previous work,19 a low ionic strength of 10 μM KCl vastly improved the ordering of attractive binary colloidal structures grown layer-by-layer (LbL). Unlike the uniform negative potential of a bare glass substrate, the underlying negative L-colloidal template in the LbL case can provide an ordered potential landscape to aid in improving the order of the next layer. The oppositely charged S-colloids are postulated to possess maximum mobility to configure into a highly symmetrical LS2 2D-superlattice under those conditions. However, both lower and higher density binary structures are also present. We repeated the LbL experiments at three evaporation temperatures: 25, 30, and 35 C. Figure 5 shows that the crystalline quality strongly depends on the evaporation temperature and the most uniform LS2-superlattice density was found to form at 25 C. The lower evaporation rate and slower crystal growth rate at 25 C allow the S-particles more time to stabilize and order into thermodynamically favorable LS2-superlattice in-plane in (41) Nelson, E. C.; Braun, P. V. Science 2007, 318, 924–925.
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Figure 5. Growth of LS2 superlattice at (a, b) 25, (c) 30, and (d) 35 C at various magnifications. The highest density of well-defined LS2
superlattice is found under the conditions of lower evaporation temperature of 25 C. (b) The LS2 schematic (drawn to scale) in the inset panel shows the L-colloidal template could only accommodate five S-particles at the interstitial sites for the size ratio dS/dL = 0.614.
the suspension phase before settling onto the oppositely charged template. However, at higher temperatures, the ordering is disturbed due to the faster evaporation of water and greater random motion of particles, resulting in the formation of closer-packed and possibly disordered binary arrays. 3.3. Self-Assembly Mechanism of Positive Monolayer Colloidal Crystals on Negative Glass. In vertical deposition, particles are carried by the solvent flux to the meniscus edge where evaporation rate is the highest.42,43 As more colloids arrive and the local volume fraction reaches the threshold to crystallize,44,45 the interparticle interactions provide the driving force to form a small cluster of hexagonal ordered monolayer.46 This is the onset of nucleation. As drying proceeds, there is a buildup of immersion capillary forces (42) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26–26. (43) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827–829. (44) Pusey, P. N.; van Megen, W. Nature 1986, 320, 340–342. (45) Sirota, E. B.; Ou-Yang, H. D.; Sinha, S. K.; Chaikin, P. M.; Axe, J. D.; Fujii, Y. Phys. Rev. Lett. 1989, 62, 1524–1527. (46) Koh, Y. K.; Yip, C.-H.; Chiang, Y.-M.; Wong, C. C. Langmuir 2008, 24, 5245–5248.
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while the 2D crystal grows with the continuous flow of particles to the meniscus edge. However, the nucleation process of hcp monolayer array is made difficult by the presence of strong electrostatic attractions between the oppositely charged particles and substrate. We believe an intricate balance of the particulate mobility by all three parameters, (1) evaporation temperature, (2) volume fraction, and (3) ionic strength, is necessary to order the particles. For a well-ordered hcp monolayer with high crystallinity and minimum defects, the delivery speed of the particles should equal the crystal growth rate. The evaporation temperature controls both the crystal growth and particle flux rates. First, this temperature must be sufficiently low to decrease the evaporation rate and hence slow down the crystal growth. This will provide the spheres more time to organize into a favorable in-plane hcp lattice arrangement. An additional advantage is the particle kinetic mobility is also reduced to restrain the Brownian free motion from interrupting the ordering stage. Second, the number of the particles and rate of delivery to the growth front must be sufficient small so as not to overwhelm and disrupt the crystallization process. In the attractive system of positive colloids on negative glass substrate, this could be Langmuir 2010, 26(10), 7093–7100
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Figure 6. (a) DLVO interaction energy calculations of the positive PS colloids with different ionic strengths at 25 C. (b) Schematic of attractive monolayer colloidal crystal self-assembly process (not drawn to scale). High-density compact hcp monolayer arrays are grown ideally when φ e 0.05 vol % and ci e 10 μM KCl at 25 C. However, at higher volume fraction or evaporation temperature, more particles are delivered to the meniscus edge and kinetically trapped into locally ordered arrays due to the faster crystal growth rate. Aggregation of the colloids occurs at high ionic strength conditions. Langmuir 2010, 26(10), 7093–7100
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achieved by a combination of a relatively low bulk volume fraction and low evaporation temperature. Moreover, a smaller starting colloidal concentration also implies a longer time scale is needed to reach the threshold volume fraction to crystallize. This would give the particles more time to explore and arrange into favorable lattice sites. In our experiments, we determine these conditions are φ e 0.05 vol % at 25 C. Electrostatic interactions could be enrolled to further improve the uniformity and density of hcp monolayer colloidal crystals. The calculated DLVO interaction energy based on van der Waals interactions and Yukawa potentials14 for the positive PS colloids at 25 C is shown in Figure 6a. At a low ionic strength (ci = 10 μM KCl), the particles are able to approach one another closer and self-organize into a geometrically favorable hcp arrangement with the lowering of the energy barrier and reduction in the Debye screening length. The shorter range nature of the particle-glass attraction also implies the colloids have additional time to arrange before settling onto the substrate compared to the deionized condition. The most uniform density of long-ranged compact hcp domains are attained at φ = 0.01 vol % with 10 μM KCl at 25 C. However, when the KCl concentration is further increased to 1000 μM, the spheres approach too closely to one another and agglomerate. The larger sized aggregates lose the mobility to rearrange and adhere onto the glass surface in random disorder. Therefore, low ionic strengths are necessary for the colloids to develop the repulsion between the nearest neighbors essential for structural ordering.14,23 The impact of the various parameter
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effects in the self-assembly process of attractive colloidal crystals on glass is illustrated in Figure 6b.
4. Conclusion We have examined how we could optimize the mobility of charged colloids by controlling the ionic strength, volume fraction, and evaporation temperature within a challenging environment of an attractive substrate to obtain well-ordered colloidal crystals. The ideal maximum mobility of the spheres to configure into regular hexagonal close-packed monolayer arrays and welldefined 2D LS2-superlattices are found at a relatively low volume fraction (φ e 0.05 vol %), low ionic strength (ci e 10 μM KCl), and low evaporation temperature (25 C) conditions. Being able to grow well-ordered colloidal arrays onto oppositely charged substrates could substantially relax the constraints of colloidal crystal growth imposed by electrostatic limitations. Acknowledgment. We thank Mr. Chan-Hoe Yip for the digital photographs of the colloidal crystal specimens. We also gratefully acknowledge the Singapore-MIT Alliance for financial support of this work and a graduate fellowship for K.W.T. Supporting Information Available: SEM micrographs and photographs of positively charged polystyrene colloids on negative glass substrates. This material is available free of charge via the Internet at http://pubs.acs.org.
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